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By Mohsen Shahinpoor, Hans-Jörg Schneider
The Royal Society of ChemistryCopyright © 2008 The Royal Society of Chemistry
All rights reserved.
Chemically Driven Artificial Molecular Machines
JAMES D. CROWLEY, EUAN R. KAY AND DAVID A. LEIGH
School of Chemistry, University of Edinburgh, The King's Buildings, West Mains Road, Edinburgh EH9 3JJ, UK
1.1 Design Principles for Molecular-Level Motors and Machines
The widespread use of molecular-level motion in key natural processes suggests that great rewards could come from bridging the gap between the present generation of synthetic molecular systems – which by and large rely upon electronic and chemical effects to carry out their functions – and the machines of the macroscopic world, which utilise the synchronised movements of smaller parts to perform tasks. It is only in the last few years that it has become feasible to design and synthesise molecules in which well-defined large-amplitude or directional stimuli-induced positional changes of submolecular components can occur. Even so, all but the simplest questions remain unanswered. What are the structural features necessary for molecules to use directional displacements to repetitively do work? How can we make a synthetic molecular machine that pumps ions to reverse a concentration gradient, say, or moves itself energetically uphill? How can we make nanoscale structures that traverse a predefined path across a surface or down a track, responding to the nature of their environment to change direction?
Artificial compounds that can do such things have yet to be realised. Synthetic molecular machines remain very much in their infancy in terms of experimental systems and only the most basic types – mechanical switches, memories and slightly more sophisticated, but still rudimentary, motors – have been made thus far. Here, we outline the early successes in taming molecular-level movement, the underlying principles that experimental designs must follow, and the progress made towards utilising synthetic molecular structures to perform tasks using mechanical motion. We also highlight some of the issues and challenges that still need to be overcome.
1.1.1 The Effects of Scale
The path towards synthetic molecular machines can be traced back nearly two centuries to the observation of effects that pointed directly to the random motion experienced by all molecular-scale objects. In 1827, the Scottish botanist Robert Brown noted through his microscope the incessant, haphazard motion of tiny particles within translucent pollen grains suspended in water. An explanation of the phenomenon – now known as Brownian motion or movement – was provided by Einstein in one of his three celebrated papers of 1905 and was proven experimentally by Perrin over the next decade. Scientists have been fascinated by the implications of the stochastic nature of molecular-level motion ever since. The random thermal fluctuations experienced by molecules dominate mechanical behaviour in the molecular world. Even the most efficient nanoscale machines are swamped by its effect. A typical motor protein consumes ATP fuel at a rate of 100–1000 molecules every second, corresponding to a maximum possible power output in the region 10-16 to 10-8W per molecule. When compared with the random environmental buffeting of ~10-8W experienced by molecules in solution at room t emperature, it seems remarkable that any form of controlled motion is possible!
When designing molecular machines it is important to remember that the presence of Brownian motion is a consequence of scale, not of the nature of the surroundings. It cannot be avoided by putting a molecular-level structure in a near-vacuum for example. Although there would be few random collisions to set such a Brownian particle in motion, equally there would be little viscosity to slow it down. These effects always cancel each other out and as long as a temperature for an object can be defined, it will undergo Brownian motion appropriate to that temperature (which determines the kinetic energy of the particle). In the absence of any other molecules, heat would still be transmitted from the hot walls of the container to the particle by electromagnetic radiation, the random emission and absorption of the photons producing the Brownian motion. In fact, even temperature is not a particularly effective modulator of Brownian motion since the velocity of the particles depends on the square root of the temperature. So to reduce random thermal fluctuations to 10% of the amount present at room temperature, one would have to drop the temperature from 300 K to 3K. It seems sensible, therefore, to try to utilise Brownian motion when designing molecular machines rather than make structures that have to fight against it. Indeed, the question of how to (and whether it is even possible to) harness the inherent random motion present at small length scales to generate motion and do work at larger length scales has vexed scientists for some considerable time.
1.1.2 Machines that Operate at Low Reynolds Number
Whilst rectifying Brownian motion may provide the key to powering molecular-level machines, it tells us nothing about how that power can be used to perform tasks at the nanoscale and what tiny mechanical machines can and cannot be expected to do. The constant presence of Brownian motion is not the only distinction between motion at the molecular level and in the macroscopic world. In the macroscopic world, the equations of motion are governed by inertial terms (dependent on mass). Viscous forces (dependent on particle dimensions) dampen motion by converting kinetic energy into heat, and objects do not move until provided with specific energy to do so. In a macroscopic machine this is often provided through a directional force when work is done to move mechanical components in a particular way. As objects become less massive and smaller in dimension, inertial terms decrease in importance and viscosity begins to dominate. A parameter that quantifies this effect is Reynolds number (R) – essentially the ratio of inertial to viscous forces – given by equation (1.1) for a particle of length dimension a, moving at velocity v, in a medium with viscosity η and density ρ.
R = avρ/η (1.1)
Size affects modes of motion long before we reach the nanoscale. Even at the mesoscopic level of bacteria (length dimensions ~10-5m), viscous forces dominate. At the molecular level, the Reynolds number is extremely low (except at low pressures in the gas phase or, possibly, in the free volume within rigid frameworks in the solid state) and the result is that molecules, or their components, cannot be given a one-off "push" in the macroscopic sense – momentum is irrelevant. The motion of a molecular-level object is determined entirely by the forces acting on it at that particular instant – whether they be externally applied forces, viscosity or random thermal perturbations and Brownian motion. Since the physics that governs mechanical dynamic processes in the two size regimes is completely different, macroscopic and nanoscale motors require fundamentally different mechanisms for controlled transport or propulsion. Moreover, the high surface area: volume ratios of molecules mean they are inherently sticky and this will have a profound effect on how molecular-sized machines are organised and interact with one another. In general terms, this analysis leads to a central tenet: while the macroscopic machines we encounter in everyday life may provide the inspiration for what we might like molecular machines to achieve, drawing too close an analogy for how they might do it is likely to be a poor design strategy. The 'rules of the game' at large and small length scales are simply too different.
1.1.3 Lessons to Learn from Biological Motors and Machines
Help is at hand, however, because despite all these problems we know that motors and machines at the molecular level are conceptually feasible – they are already all around us. Nature has developed a working molecular nanotechnology that it employs to astonishing effect in virtually every significant biological process. Appreciating in general terms how nature has overcome the issues of scale, environment, equilibrium, Brownian motion and viscosity is extremely useful for indicating general design traits for synthetic molecular machine systems and how they might be used.
There are many important differences between biological molecular machines and the man-made machines of the macroscopic world: Biological machines are soft, not rigid; they work at ambient temperatures (heat is dissipated almost instantaneously at small length scales so one cannot exploit temperature gradients); biological motors utilise chemical energy, often in the form of ATP hydrolysis or chemical gradients; they work in solution or at surfaces and operate under conditions of intrinsically high viscosity; they rely on and utilise – rather than oppose – Brownian motion; since their components are constantly in motion, biomolecular machines need to control their directionality of movement not power their movement; the molecular machine and the substrate(s) it is acting upon are kinetically associated during the operation of the machine; biological machines are made by a combination of multiple parallel synthesis and self-assembly; their operation is governed by noncovalent interactions; and, finally, they utilise architectures (e.g. tracks) which restrict most of the degrees of freedom of the machine components and/or the substrate(s) it is acting upon.
If biology and physics provide the inspiration and strategies for controlling molecular-level motion, it is through chemistry that artificial molecular-level machine mechanisms must be designed, constructed and made to work. The minimum requirements for such systems must be the restriction of the 3D motion of the machine components and/or the substrate and a change in their relative positions induced by an input of energy. Methods for achieving this are described in the following sections.
1.2 Controlling Motion in Covalently Bonded Molecular Systems
1.2.1 Controlling Conformational Changes
22.214.171.124 Stimuli-induced Conformational Control around a Single Covalent Bond
As a first step towards achieving controlled and externally initiated rotation around C-C single bonds, Kelly combined triptycene structures with a molecular recognition event. In the resulting 'molecular brake', 1 (Scheme 1.1), free rotation of a triptycyl group is halted by the conformational change brought about by complexation of the appended bipyridyl unit with Hg2+ ions – effectively putting a "stick" in the "spokes".
Kelly and coworkers then extended their investigation of restricted Brownian rotary motion to a molecular realisation (2) of the Feynman adiabatic Ratchet-and-Pawl in which a helicene plays the role of the pawl in attempting to direct the rotation of the attached triptycene "cog-wheel" in one direction owing to the chiral helical structure. Although the calculated energetics for rotation showed an asymmetric potential energy profile (Figure 1.1(b)), 1H nuclear magnetic resonance (NMR) experiments confirmed that rotation occurred with equal frequency in both directions. This result is, of course, in line with the conclusions of the famous Feynman thought experiment. The rate of a molecular transformation – clockwise and anticlockwise rotation in 2 included – depends on the energy of the transition state (and the temperature), not the shape of the energy barrier – state functions such as enthalpy and free energy do not depend on a system's history. Thus, although rotation in 2 follows an asymmetric potential energy surface, at equilibrium, the principle of detailed balance requires that transitions in each direction occur at equal rates.
The essential element missing from 2 needed to turn the triptycene direc-tionally is some form of energy input to drive it away from equilibrium and break detailed balance. Kelly proposed a modified version of the ratchet structure, 3a (Scheme 1.2), in which a chemical reaction is used as the source of energy. Ignoring the amino group, all three energy minima for the position of the helicene with respect to the triptycene "teeth" are identical – the energy profile for 360° rotation would appear as three equal energy minima, separated by equal barriers. As the helicene oscillates back and forth in a trough, however, sometimes it will come close enough to the amine for a chemical reaction to occur (as in 4b). Priming the system with a chemical "fuel" (phosgene in this case to give the isocyanate 4a), results in "ratcheting" of the motion some way up the energy barrier (5a). Continuation of the rotation in the same direction, over the energy barrier can occur under thermal control and is now an exergonic process (giving 5b) before cleavage of the urethane gives the 120° rotated system (3b). Although the current system can only carry out one third of a full rotation, it demonstrates the principles required for a fully operating and cyclable rotary system under chemical control and represents a major advance in the experimental realisation of molecular-level machines.
Feringa and coworkers have successfully adopted a strategy based on the stereoselective ring-opening of racemic biaryl-lactones using chiral reagents to obtain a full 360° unidirection rotation around a C-C single bond. The process involves four intermediates (A-D, Figure 1.2(a)), in each of which rotation around the biaryl bond is restricted: by covalent attachment in A and C; and through nonbonded interactions in B and D. Directional rotation to interchange these intermediates requires a stereoselective bond-breaking reaction in steps (i) and (iii) and a regioselective bond-formation reaction in steps (ii) and (iv). The lactones 6 and 8 (Figure 1.2(b)) exist as racemic mixtures due to a low barrier for small amplitude rotations around the aryl-aryl bond. Reductive ring opening with high enantioselectivity is, however, achievable for either lactone, using a homochiral borolidine catalyst and the released phenol can subsequently be orthogonally protected to give 7a or 9a. The ring-open compounds are produced in near-enantiopure form in a process that involves directional rotation of 901 around the biaryl bond, governed by the chirality of the catalyst, and powered by consumption of borane. The ortho-substitution of these species results in a high barrier to axial rotation. Oxidation of the benzylic alcohol (7a [right arrow] 7b or 9a [right arrow] 9b) primes the motor for the next rotational step. Selective removal of one of the protecting groups on the enantiotopic phenols results in spontaneous lactonisation when thermally driven axial rotation brings the two reactive groups together – again probably a net directional process owing to the steric hindrance of the ortho-substituents (although this is not demonstrated because the chirality is destroyed in this step). Figure 1.2 illustrates the unidirectional process achieved using the (S)-CBS catalyst, rotation in the opposite sense can be achieved by employing the opposite borolidine enantiomer and swapping the order of phenol protection and deprotection steps.
Excerpted from Intelligent Materials by Mohsen Shahinpoor, Hans-Jörg Schneider. Copyright © 2008 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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